If you download the newest version of Firefox, it comes with an application to do just that. Not only xvideos, but youtube.... pretty much anywhere you can see a video on the internet. It will be the little button right under the x to close the Firefox window. It has a little arrow. All you have to do is, click on that arrow and the name of the video and its size will show up. just click on the name and it will start downloading. It will save it in your downloads folder if you are a Windows user. Let me know if it works!